1. Field of the Invention
The present invention generally relates to the field of optical fibers and to manufacturing methods thereof. More particularly, the invention concerns an optical fiber link featuring a low Polarization Mode Dispersion (shortly, PMD), and a method of realizing it.
2. Description of the Related Art
Optical signals transmitted through single-mode optical fibers comprise two orthogonal polarization modes (typically denoted Transverse Electric, or TE, and Transverse Magnetic, or TM). In case the fiber has a perfectly cylindrical core of uniform diameter, the two modes TE and TM propagate at a common velocity. However, in real optical fibers the cylindrical symmetry of the core may be disrupted due to shape defects or non-uniform stresses. As a result, a phase difference can accumulate between the two modes as they propagate, and the fiber is said to exhibit “birefringence”. In particular, the birefringence introduced by shape and stress asymmetry is known as “intrinsic linear birefringence”.
The structural and geometrical irregularities of the optical fiber that give rise to birefringence typically originate from the fiber preform itself, and are modified during the process of drawing the fiber. This process is usually carried out by means of an apparatus known as a “drawing tower”, starting from a glass preform. In practice, after the preform has been placed in vertical position and heated to a temperature above the softening point within a suitable furnace, the molten material is drawn downwards at a controlled velocity in such a way as to produce a threadlike element that forms the optical fiber itself. In this process, asymmetrical stresses are typically applied to the fiber.
In a birefringent fiber, the two components TE and TM of the fundamental optical mode, initially in phase with each other, return to be in phase again only after a certain propagation length, commonly known as the “beat length” (LB). In other words, the beat length is the period of repetition of a certain state of polarization (on the assumption that the fiber maintains a constant birefringence over this length). A further characteristic parameter of a birefringent fiber is the “correlation length” (LF), which is defined as the distance over which the autocorrelation function of the birefringence is 1/e times its maximum value.
In the so-called “polarization-preserving” optical fibers, asymmetry is deliberately introduced into the fiber to generate birefringence. However, in ordinary (i.e., non-polarization-preserving) fibers, birefringence is detrimental to the fiber performance.
In fact, when pulsed signals are transmitted into an optical fiber, birefringence is a cause of pulse spreading, since the two polarization components, TE and TM, travel at different group velocities (i.e. become dispersed). This phenomenon, known as Polarization Mode Dispersion (PMD), has been widely studied in recent years because of its importance in periodically amplified light guide systems.
Typically, the phenomenon of PMD leads to a limitation of the width of the signal transmission band and, consequently, a degradation of the performance of the optical fibers along which the aforesaid signals are transmitted. This phenomenon is therefore undesirable in systems of signal transmission along optical fibers, especially in those operating over long distances, in which it is necessary to minimize any form of attenuation or dispersion of the signals to guarantee high performances in transmission and reception.
U.K. patent application GB-A-2101762 considers the effects on PMD of post-draw fiber twisting and observes that, although such twisting reduces the PMD resulting from intrinsic linear birefringence, it introduces torsional stresses that generate a substantial circular birefringence due to the photo-elastic effect. Twisting a drawn fiber thus reduces the bandwidth limitation due to one effect, whilst replacing it with another. The same patent application thus proposes to spin the preform during drawing, so that twisting can be effected whilst keeping the fiber material substantially unstressed. Spinning is performed at a relatively high rate, so that its spatial repetition frequency, or spin pitch, is small compared to the beat length due to intrinsic birefringence; as a result, an optical fiber can be produced wherein the contribution of birefringence due to form and stress asymmetries is greatly reduced. Such a fiber is termed “spun” fiber, to distinguish it from a (post-drawn) twisted fiber. Conveniently, the preform is spun at a substantially constant rate, but it could even reverse in direction, oscillating from a right-handed to a left-handed twist.
In the present description, the same distinction as above will be made between “spin” and “twist”. More precisely, the terms “spin” and “twist” are herein used to identify two different types of torsion of the fiber: “spin” identifies a torsion that is frozen-in during drawing, being applied to a viscous portion of the fiber and kept as a structural modification of the fiber while cooling; differently, “twist” identifies an elastic torsion of the fiber, which is present when a torque is applied to a portion of fiber whose ends are constrained against rotation. In other words, although both spin and twist alter the fiber in shape, so that parts previously in the same straight line are located in a spiral curve, a twisted fiber will rotate back to its original shape when its ends are released from the rotation constraint, while a spun fiber will keep this alteration as an intrinsic and permanent deformation. Due to spinning, the fiber undergoes a rotation of its polarization axes. As a result, when optical pulses are transmitted into the optical fiber, they propagate alternately on the slow and fast birefringence axes, thus compensating the relative delay and reducing the pulse spreading. This is equivalent to having a local effective refractive index for the optical pulses equal to the mean refractive index on the two axes, the average being taken over the pulse length along the fiber.
Theoretical studies have shown that the dominant process for the reduction of PMD in a spun fiber is the averaging of the local fiber anisotropy by the rapid procession of the axes of asymmetry along the fiber.
The U.S. Pat. No. 4,504,300, relating to a technique for making an optical fiber having chiralic structure, addresses drawbacks related to preform rotation and proposes a new spinning technique, consisting in rotating the fiber instead of the preform. In particular, a device is disclosed comprising means disposed just below the preform for twisting the fiber during fiber drawing. The twisting means comprise a rotating hoop supporting three pulleys. The twisted fiber is coated by coating means, followed by cooling by fast-cooling means that facilitate freezing-in of the twist.
The U.S. Pat. No. 5,418,881 proposes to arrange the device adapted to apply the torque to the fiber downstream of the coating station, so as to avoid damaging the fiber surface. In particular, the torque is applied by alternately canting in clockwise and counterclockwise direction a fiber-guiding roll having a rotation axis which extends perpendicularly to the drawing axis of the fiber. In this way, in at least a portion of the fiber the spin impressed to the fiber is alternately clockwise and counter-clockwise. The same patent states that applying a clockwise and a counterclockwise torque to the fiber substantially prevents introduction of an elastic twist to the fiber.
The United States patent application N. US2001/0020374 proposes a new device that overcomes some drawbacks of the canting-roll technique and allows both unidirectional and alternate spinning, but also states that alternate spinning is to be considered as preferable since it prevents the presence of residual torsions (i.e., of a residual twist) on the fibers wound onto the collecting spool, thus making easier both the unwinding and wiring operations of the same.
In the U.S. Pat. No. 5,943,466, it is proposed to spin the fiber during drawing in accordance with spin functions which are not substantially constant (in the sense that they change substantially as a function of distance along the length of a fiber or as a function of time), not substantially sinusoidal, and have sufficient variability (e.g. sufficient harmonic content) to provide a substantial reduction in PMD for a plurality of beat lengths.
The Applicant has found some other drawbacks of the alternate spinning technique, not previously highlighted. Alternate spinning may for example cause a relatively low mechanical efficiency of the spinning device, due to the continuous accelerations and decelerations. Moreover, with respect to a unidirectional spin, an alternate spin requires a relatively high peak profile amplitude to compensate those positions of the profile where the rotation slows down to change direction and, therefore, to guarantee a sufficient average spin rate. Besides all this, the sites where the spin rate is zero are detrimental for the PMD, because there is an increase of the effective birefringence seen by the pulse, and so a higher contribution for PMD.
The paper by A. Galtarossa et al., “PMD statistical properties of constantly-spun fibers”, ECOC-IOOC 2003 Proceedings, Vol. 4, Th. 1.7.4, and the paper by A. Galtarossa et al. “Polarization mode dispersion properties of constantly spun randomly birefringent fibers”, Optics Letters, vol 28 No. 18, September 2003, pp. 1639-1641 report the PMD induced delay (i.e. the mode delay—in ps—induced by PMD or, equivalently, the mean fiber Differential Group Delay, or “DGD”) of unidirectionally-spun fibers. It can be shown that, while in an unspun fiber or an alternately spun fiber the PMD induced delay increases proportionally to the square root of the fiber length, in a unidirectionally-spun fiber the PMD induced delay has a higher increase rate, and only asymptotically increases proportionally to the square root of length. In particular, the PMD induced delay in a unidirectionally-spun fiber asymptotically increases at the same rate as the PMD induced delay of an unspun fiber having the same beat length LB and the same correlation length LF. Advantageously, a PMD coefficient, hereinafter indicated with PMDc, defined as the mean fiber DGD divided by the square root of length, is introduced. For unspun or alternately spun fibers, this parameter is independent from the fiber length.
In greater detail, reference is made to FIG. 1, wherein a theoretical diagram of the average of the squared DGD <Δτ2> (in ordinate, unit ps2) as a function of the propagation distance (in abscissa, unit km) is shown for an unspun fiber (curve (a)) with a typical (constant) PMDc (e.g., 0.1 ps/km1/2), an alternately spun fiber (curve (b)) with a typical (constant) PMDc (e.g., 0.04 ps/km1/2) and a unidirectionally spun fiber (curve (c)) with the same beat length LB and the same correlation length LF as the unspun fiber. From the diagram, it can be appreciated that the slope of curve (c) (i.e. the increase rate of <Δτ2>) is not constant, but increases with the propagation distance up to a constant value corresponding to the slope of curve (a). The length over which the slope changes can be denoted as a transient length. Since the PMDc is proportional to the square root of <Δτ2> divided by the square root of the fiber length, it is expected that such a coefficient increases with the propagation distance (i.e. with the fiber length), differently from the PMDc of unspun and alternately spun fibers, which is constant. In particular, for the unidirectionally spun fiber, the increase of the PMDc will be more rapid in the initial transient, before the increase rate of the PMDc becomes similar to that of the unspun fiber; after the transitory, the PMDc increases very slowly reaching asymptotically the PMDc of the unspun fiber. As already predicted in the article by Galtarossa et al., “Optimized Spinning Design for Low PMD Fibers: An Analytical Approach” Journal of Lightwave technology vol. 19 no. 10 October 2001 pp. 1502-1512, the initial PMDc increase is the one predicted in the deterministic regime.
In the above-cited articles by Galtarossa, it is also described that the magnitude of the spin period changes the length of the above-mentioned transient regime, and that a transient characteristic length LT can be defined for unidirectionally spun fibers (curve (c) in FIG. 1):
      L    T    =            L      F        ⁡          (              1        +                              4            ⁢                                                  ⁢                          L              B              2                                            p            2                              )      where p is the spinning period, LF the correlation length and LB the beat length. The transient characteristic length LT is equal to the intercept of the linear asymptotic behavior of curve (c) with the abscissa axis. The propagation distance (or length of fiber span) required to approach the regime PMD behavior of the unspun fiber is estimated to be of some transient characteristic lengths.
Assuming that the parameters appearing in the above formula fall within the typical ranges: LF=1÷20 m, LB=5÷15 m, and p=0.1÷1 m, the transient characteristic length LT may vary between 0.1 and 1,800 km, covering four orders of magnitude. If the transient characteristic length LT is much greater than the link length, the PMDc increase remains moderate. On the contrary, when the transient characteristic length LT is comparable to or smaller than the link length, the PMDc increase over the link becomes significant and can be detrimental to signal transmission.
Thus, unidirectionally spun fibers with short transient characteristic lengths suffer from a growth of the PMDc with the fiber length, which cancel the advantage of using a spun fiber.
Another prediction made in the cited paper by A. Galtarossa published in Optics Letter is that the DGD statistical distribution for short enough unidirectionally spun fibers may deviate from the typical Maxwell distribution exhibited by both unspun and alternately spun fibers.